The following contains spoilers for X-Men Red #5, now on sale from Marvel Comics.
The mutants of the Marvel Universe have been targeted frequently over the years, forced to contend with truly fearsome and insidious forces. They often have to do so without the assistance of their nominal allies -- but the latest salvo against the X-Men as a part of A.X.E.: Judgement Day might force Earth's Mightiest Heroes to finally choose a side.
The slaughter of Arakko by Uranos (and by extension the Eternals) in X-Men Red #5 (by Al Ewing, Stefano Caselli, Federico Blee, and VC's Ariana Maher) makes any peace between them and the mutants tricky. And the Avengers are awkwardly stuck right in the middle of a truly unwinnable situation as a result.
Over the course of Marvel's history, the Avengers and the X-Men have always had a tumultuous relationship. While there are friendships between the two groups and periods of extended cooperation between them (like when Rogue led the Avengers during the formation of the Unity Squad), the two groups have also fought plenty of times. A major sticking point has been and remains to be the fate of Genosha, where over sixteen million mutants were wiped out by Sentinels commanded by Cassandra Nova in the "E is for Extinction" storyline from New X-Men #114-116 (by Grant Morrison and Frank Quitely).
The Avengers and the world at large did little to assist the mutants in the aftermath of that massacre, something Emma Frost has reminded the world of repeatedly. Now, the Avengers are faced with a similar event. As part of his overt offensive against the mutant nations, Druig unleashed the terrifying Uranos onto Arakko. The villain was able to wipe out roughly 98% of the planet's population, depleting the excess number of mutants in the universe. Already, the Krakoan Quiet Council is preparing a rebuttal -- and Magneto (one of the few survivors) seems poised to bring vengeance upon the Eternals for their actions. On top of all this, Druig has already announced his intentions to humanity at large -- seemingly convincing crowds of regular people that the Eternals are in the moral right to be targeting mutants and jump-starting a conflict with them that could radically alter the fate of the Marvel Universe.
As the conflict escalates, the Avengers find themselves trapped between two extremes. At the moment, their plan seems to involve working with the rogue Eternals like Sersi, Ajak, and Makkari to create a new Celestial. But eventually, the Eternals will be forced to take a side in the conflict. The slaughter of Arakko will likely not be forgotten by a mutant population that spent years despising the Scarlet Witch for her role in just depowering mutants. If the Avengers endeavor to help the X-Men find justice for these acts, they risk turning the Eternals against them -- endangering their plan and potentially drawing public scrutiny. But if they choose to look past the deaths of hundreds of thousands of mutants by the hand of the Eternals, then the X-Men and their allies will likely turn further inward. This, in turn, will escalate the already existing tensions between the mutant community and the rest of the Marvel heroes.
Even just trying to serve as peace brokers might enrage both sides -- as the Eternals are seemingly driven to eradicate mutants, while the X-Men are fighting an enemy that wiped out scores of their people. Given the global stakes of any conflict with the Eternals (as it likely endangers the Machine i.e. the Earth itself), the Avengers are eventually going to have to pick a side in this conflict. They will either appease the mutants' justifiable anger towards the Eternals or make themselves complacent in the slaughter by doing nothing. Neither will be easy -- and working with the Eternals who kidnaped a member of the Krakoan government isn't going to do the Avengers any favors. Even creating a new Celestial to halt the hostilities likely won't settle these matters, and the impact of this slaughter might reverberate even once the war is finally over.